The present invention relates to the field of mixers for fluids.
In particular, the present invention has been developed with reference to a mixer for fluids disposed in containers, of the so-called vibration type in which an inner frame which supports the container is set in oscillating motion along a predetermined path relative to the outer frame.
A vibration mixer of the known type indicated above is described in GB-1 310 655. Although this mixer has basic components for bringing about a vibratory mixing motion, it is bulky and not easy for an operator to use since it has means for clamping the container, consisting of a manually-operated driving screw which does not permit precise detection of the force with which the container is clamped on the machine. Moreover, the container has to be positioned in the machine manually and this operation may be difficult or onerous if not impossible, particularly with large or heavy containers. Moreover, the machine of the prior art has no protection or safety systems for protecting the operator during mixing operations, particularly when the movement of large and heavy containers gives rise to large forces and moments of inertia.
Another type of vibration mixer is described in the Applicant's European patent EP-0 617 998. This mixer is considerably improved in comparison with previously known mixers both from the point of view of mixing efficiency and as regards bulk, ease of use and the protection and safety of the operator.
However, the increasing need for mixing machines which are reliable, efficient and easy to use and maintain has led to a search for solutions to problems the awareness of which is greater and greater in the field.
U.S. Pat. No. 4,134,689 describes a mixing machine comprising a unit for clamping fluid containers in which two opposed clamping plates are opened and closed by the rotation of two vertical driving screws. During the mixing motion, the forces and moments of inertia generated by the mass of the container and of the fluids contained therein are discharged to the driving screws. This situation has two consequences: on the one hand, the continual stressing of the threads of the driving screws leads to rapid wear thereof and to an increase in play such as to render the reliability of the clamping of the container unacceptable and make it necessary to repair or replace the screws frequently and, on the other hand, bending of the driving screws, which is not preventable beforehand, takes place during the vibratory motion of the mixer, and, as well as rendering the methods of controlling the clamping force unreliable, necessitates the provision of oversized screws with the application of high safety factors to take account of unknown stresses. In any case, the bending stress on the driving screws is also undesirable because the flexural strength of the driving screws is reduced both by the accompanying tensile stresses due to the clamping of the container and by the presence of the threads which generate localized stresses which may reach very high levels.
In some mixing machines of known type it is possible to remove the lower plate at least partially to facilitate the loading of the container into the machine and its unloading therefrom. The systems currently used are not very reliable, however, since, on the one hand, an increase in the play of the members for ensuring the sliding of the lower support plate of the container is easily brought about with consequent adverse effects on the control of the clamping force of the container in operative conditions, on the other hand, sediments, fluid particles or, in any case, deposits may block the normal movement to extract the plate.